Electric heating unit



March 18, 1930. C us ov 1,751,416

ELECTRIC HEATING UNIT Filed March 4, 1926 lnventor-z John C. Musg-rove,

his Agborheg.

.45 ment 11 is Patented Mar. 18, 1930 UNITED STATES PATENT JOHN G.MUSGROVE, LANESBORO, MASSACHUSETTS, ASSIGNOR TO GENERAL ELEG- TRICCOMPANY, A CORPORATION OF NEW YORK ELECTRIC HEATING UNIT Applicationfiled March 4, 1926. Serial No. 92,280.

My invention relates to electric heating units and has for its objectthe provision of a simple, reliable and efiicient device of thischaracter.

5 More specifically my invention relates to electric heating units ofthe type provided with a protective sheath, although it has obviousapplication to various other types of electric heating units. In oneform of 1 sheathed heating unit the resistance conductor is embedded inelectrically insulating heat refractory material inside a metallicsheath. In one construction, the resistance conductor is given a sinuousform, such as the form of a helix, as described and claimed in Patent N0. 1,367,341 to Abbott dated February 1, 1921. his resistance conductorextends substantially throughout the full length of the sheath and hasits ends secured to metallic terminal members which project from thesheath.

In one of its aspects my invention relates to the provision of aresistance element having relatively low resistance but made ofrelatively small wire, and to that end I provide in 26 one form of myinvention a plurality of helical resistance conductors having asubstantially common axis and arranged with their turns alternating.

For a more complete understanding of my an invention reference should behad to the accompanying drawing in which Fig. 1 is a fragmentary view ofa partially constructed electric heating unit,embodying my invention;Fig. 2 is a sectional View along the line 2-2 of Fig. 1 looking in thedirection of the arrows; Fig. 3 is a fragmentary view partially insection of an electric heating unit embodying my invention; while Figs.1, 5 and. 6 are views showing details of construction.

Referring to the drawing, embodying my invention form cylindrical, inwhich a resistance heating eleembedded in powdered heat refractoryinsulating materialf'1'2, such as magnesium oxide. The resistanceelement 11 is a heating unit comprises in one Wound helicallyand extendssubstantially.

throughout the length of the sheath. It is electrically connec ed at itsends with metallic an outer metallic sheath 10, preferably Th terminalmembers 13 and 14: which project from the ends of the sheath.Preferably, the insulating material 12 is compacted by reducing thediameter of the sheath 10, for example, by swaging.

Any suitable connection may be used between the terminal members 13 and14 and the resistance element whereby good electrical and mechanicalconnections are assured. Preferably, however, the terminal members, asindicated at Fig. 4, are cylindrical in form and of such size at theirinner ends that these inner ends can be inserted easily in the ends ofthe helix of the resistance element, i. e., inserted in a plurality ofturns of the helix at each end. After being thus inserted in the helixeach terminal is secured in a suitable manner. As shown, the turns ofthe resistance conductor surrounding the inner end of each terminal aresecured thereto by compressing these turns, for example, by swaging, asdescribed and claimed in a copending application of Charles C. Abbottand George D. Milnes, Serial N o. 7 9,878, filed January 7, 1926. Thisswaging has the effect of forcing the end turns of the resistanceelement into the metal of the terminal and a good mechanical as Well. aselectrical connection is thus formed. The partial embedding of the turnsof the resistance element is clearly shown in Fig. l in which the bareinner end of the terminal 13 is shown after the compressing or swagingoperation. Preferably pressure to secure the end turns to the terminalis applied at a number of spaced points around the circumference of theturns, for example,- at diametrically opposite points along twodiameters at right angles to each other, as indicated in the drawing.This makes four zones of pressure around the circumference of the turns,each zone extending longitudinally with respect to the terminal. iscompressing operation may be carried out by means of tv vo pairs ofswaging jaws. If desired, however, the connections between the terminalmembers and the resistance conductor may be made by threading the innerends of the terminals and then screwing the endvturns of the resistanceelement on to these threaded portions of the terminals, as described andclaimed in Fatent No. 1,494,938 to Abbott, dated May 20,1924. I

When the terminals are inserted in the ends of the resistance elementhelix as previously described, there is a tendency, when a resistanceconductor of large diameter is'used in If such resistance conductors oflarge diameters are used therefore, it is desirable to provide a sheath10 of relatively large diameter in order that itinay not be undulyweakened duringthe swa-ging operation. The use of a relatively largesheath, however,-is an obvious disadvantage since the unit then occupiesmore space and cannot be as conveniently applied to the parts to beheated, nor as easily bent into various shapes. Another disadvantage ofusing 'a resistance conductor of largev diameter, is that the resultingstiifnessof such a unit makes ithard to assemble. I

In accordance with my invention I provide a heating element comprising aplurality of helical resistance conductors, each being connected at itsends to the terminal members. As shown in the drawing, two helically 1wound resistance conductors 11 and 11 are provided, although obviouslyany suitable number may be used. The turns of the two helixes are placedbetween each other as in-- dicated clearly in Fig. 5," and wheremorethan two helixes are used the turns will be arranged consecutivelyin asimilar manner. Preferably the heating element is formed by winding theplurality of resistance conductors together on a single arbor ormandrel, the two or more resistance conductors being placed sideeby sideand wound on the arbor I m the manner of a single. conductor. As woundthe turns of the resistanceconductor are arranged tightly togetherinengagement with each other and the end turns as shown are secured inthis relation on the terminal members. Subsequently, when the resistanceconductor is placed in the sheath, it is stretched to .separate theintermediate turns between the 'terminal members as will be hereafter.pointed out.

The unit is assembled insubstantially the manner described in theaforesaid Patent No. 1,494,938. The terminals 13 and 1.4 are firstattached to the resistance conductor and then the resistance conductorwith the terminals is inserted in the sheath. A spacing block 15 made ofa suitable refractory insulating material, such as lava, is provided oneach cated by the reference terminal, this block being square and ofsuch size that its corners engage with the inner surface of the sheath(Fig. 2), thus holding the inner end ofthe terminal in a centralposition. As shown in Fig. 4 the outer end of the terminal is somewhatlarger than the inner end portion, which is of such size as tobeinserted easily in the resistance conductor, so that a shoulder 16 isformed. The small inner end of the terminal is inserted in the he 'jcalresistance conductor until the end of the resistance conductor restsagainst the shoulder '16, and then these two parts are secured togetherby swaging as previously described. Thereafter the spacing block 15 isslipped over the larger outer end of the terminal and seated against theinner end of the resistance conductor, the outside diameter of theresistance conductor helix being somewhat greater than the diameter ofthe outer end of the terminal. The block 15 is secured in this positionby forcing cars 15 out of-the sides of the terminal." The outer end ofthe terminal is also cut away at opposite sides to form a flat portionwhich is provided with an aperture 17 Continuing with the assembly ofthe unit,

one end of the sheath is closed by 'means of a washer 18 which is seatedagainst a shoulder formed by countersinking the end of the sheath, andthe terminal at that end, indinumber 13, is secured against longitudinalmovement inward by inserting a pin 19 through-its aperture 17. The otherterminal 14 is secured by passing a pin .20 similar to pin 19 in anaperture in its outer end, this pin resting at its ends on a shoulderformed by countersinking the end of the sheath. The helix is of suchlength that when the terminals are secured in the sheath as previouslydescribed, the resistance conductor is stretched whereby its turns areseparated, as shown in the drawing.

The unit as thus far.assembled is next placed in a suitable vibratingmachine, the end closed by the washer 18 being at the bottom, andpowdered insulating material poured in at the upper end past the sidesof the pin 20. After the sheath has been filled with the insulatingmaterial it is closed by means of a washer 21, the end being crimpedover at 22 to secure the washer in place, and the opposite end may belikewise criniped as indicated by reference numeral 23 to prevent theterminals 13 from being accidentally forced outward. In the next stepthe unit, assembled as shown in Fig. 1, is passed through a suitableswaging or rolling machine whereby the insulating material is compacted.Preferably the insulating ma-' terial is compacted to a hard dense masswhich is a good electrical insulator, and also a good'conductor of heat.This swaging operation reduces the diameter of the unit and alsoincreases its length somewhat.

It will thus be observed that in the specific embodiment of my inventiondisclosed the helical resistance conductors, having equal diameters, arearranged in the sheath with the turns of each helix spaced apart andspaced from the turns of the other helix while the insulating materialis compacted in the sheath around the turns of each helix and betweentheturns of the helixes.

The pins 19 and 20 are off-set at the center as indicated in Fig. 6 soas to form a central recess 24 on one side and a corresponding extension25 on the opposite side. This construction provides for centering theouter end of the terminal 14 While the sheath is being filled withinsulating material since the fiat outer end of the terminal seatsitself in the recess 24, as shown. Another object of this irregularconstruction of the pins is to permit them to collapse readily duringthe swaging operation when the sheath is, being'reduced in diameter.

After the swaging operation the unit-is cut otf ateach end to remove thewashers and the flattened ends of the terminals, and the sheath cut backat each end to expose short lengths of the terminals after whichterminal lugs 26 and 27 are secured in the manner disclosed in theaforesaid patent to Abbott No. 1,494,938. Briefly, this operationconsists in providing nicks 28 on opposite sides of the terminals andcompressing the terminal lugs into these nicks so as to secure them,each end of the unit being closed by an insulating washer 29 againstwhich the terminal lug is seated.

lVhile I have described my invention as embodied in concrete from and asoperating in a specific manner in accordance with the provisions of thePatent Statutes, it should be understood that I do not limit myinvention thereto, since various modifications thereof will suggestthemselves to those skilled in the art without departing from the spiritof my invention, the scope of which is set forth in the annexed claims.

lVhat I claim as new and desire to secure by Letters Patent of theUnited States is:

1. An electric heating unit comprising a metallic sheath, compactedpowdered insulating material in said sheath, a pair of metallic terminalmembers projecting from-said sheath, anda plurality of sinuousresistance conductors embedded in said insulating material, each of saidresistance conductors having a plurality of turns at each endsurrounding said terminal members respectively and secured thereto.

2. An electric heating unit comprising a metallic sheath, compactedpowdered insulating material in said sheath, a pair of metallic terminalmembers projecting respectively from the ends of said sheath, and a.-plurality of helical resistance conductors of equal diameters embeddedin said insulating material, said resistance conductors being arrangedso that their axes are substantially coincident with their turns side byside in spaced relation, each of said resistance conductors having aplurality of turns at each end surrounding and secured to said terminas.

3. An electric heating unit comprising a metallic sheath, a plurality ofhelical resistance conductors arranged in said sheath with their axessubstantially coincident and with the turns of each helix spaced apartand spaced from the turns of the other helix, said helixes having equaldiameters, and powdered insulating material in said sheath compactedaround the turns of each of said helixes and between the turns of saidhelixes, metallic terminal members extending into said sheath, andconnections between the ends of said resistance conductors and saidterminals.

In witness whereof, I have hereunto set my hand this first day of March,1926.

JGHN C. MUSGROVE.

